1. Field Of The Invention
This invention relates to a helically threaded molded thermoplastic closure which is made by the unscrewing process and which incorporates a bead to retain a sealing liner in position against the underside of the top of the closure, to thereby eliminate the need for an adhesive to bond the closure liner to the underside of the closure. More particularly, this invention relates to a closure of the aforesaid character which functions as the inner member of a two-piece child-resistant closure that is used in conjunction with an outer member. The outer member fits over the inner member and cooperates with the inner member to permit the removal of the inner member by the proper manipulation of the outer member in a way which imparts child-resistant opening characteristics to the combined closure.
2. Description Of The Prior Art
As is known in the prior art, molded plastic closures with internal helical threads can be manufactured by a process in which each closure is removed from the molding tooling either by striping the closure from the tooling or by unscrewing the closure from the tooling. Generally speaking, the stripping process is faster and less expensive, but the unscrewing process, while slower and more expensive, can be used to provide the closure with superior resistance to being stripped from the threaded finish of the container to which it is ultimately applied. As is also known in the prior art, it is frequently desirable to provide a helically threaded molded plastic closure, whether made by the stripping process or by the unscrewing process, with a soft pulp or plastic sealing liner on the underside of the top of the closure to permit the closure to properly seal against the rim of the container to which it is ultimately applied. The sealing liner may be adhesively attached to the underside of the top of the closure, unless the contents to be placed into the associated container are chemically incompatible with the type of adhesive that is used in the attachment of the liner to the molded plastic closure. In cases where it is undesirable to use an adhesively attached sealing liner, it is also known that the sealing liner may be suitably retained in place by providing the inside of the skirt portion of the closure with an inwardly projecting bead which, by virtue of providing an interference fit with an outer peripheral portion of the sealing liner, mechanically or frictionally retains the sealing liner in the desired position against or adjacent to the underside of the top of the closure. U.S. Pat. No. 2,039,757 (L.A. VonTill) discloses a molded plastic closure which is made by the stripping process and which incorporates a double ended bead, described as a circumferential ledge, to retain a sealing liner in position against the underside of the top of the closure.
As is explained in the aforesaid U.S. Pat. No. 2,039,757, it is necessary to discontinue the closure liner retention bead over at least a portion of the circumference of such bead to avoid interference with the upper portion of the helical thread on the closure to which the molded plastic closure is to be affixed. Because of this problem of interference between the upper portion of the helical thread on the closure and the liner retention bead of the closure, the use of molded plastic closures with liner retention beads has heretofore largely been limited to use on containers of a type that has a non-standard finish design, that is, to containers of a type that has a finish design with a larger than standard "S" dimension, the vertical dimension between the top of the container rim and the start of the helical thread on the container. Of course, the use of any such container with a non-standard finish portion, especially with a finish portion that has greater vertical height than would be necessary with a standard finish portion, is undesirable in that it adds to the manufacturing cost of such containers and to the manufacturing cost of the closures therefor, and it adds to the cost of applying closures to filled containers on a mass production basis.
The use of a radially inwardly projecting bead for purposes of the retention of the liner of a lined closure is even more difficult in the case of a molded plastic closure which is made by the unscrewing process, since the molded plastic bead does not have a helical angle and, thus, must be stripped from the adjacent portion of the mold tooling notwithstanding the fact that the rest of the closure is being removed from such tooling by unscrewing. This fact limits the radial depth of such liner retention bead, and this limitation is especially severe in the case of closures which are made from unusally hard thermoplastic materials such as PET (polyethylene-terepthalate) or polypropylene, materials which sometimes must be used for purposes of chemical compatibility with the product packaged in the associated container, for example, bleach products and petroleum distillate products.